The present invention relates to a power transmission system in an agricultural tractor.
In an agricultural tractor, an engine is mounted on a front portion of the vehicle body, and on the rear portion of the vehicle body is mounted a transmission case. An output shaft of the engine is connected to an input shaft of the transmission case via a propeller shaft. Within the transmission case is contained a speed change mechanism, hence the engine power input to the above-mentioned input shaft is transmitted at a changed speed to an output shaft via the aforementioned speed change mechanism, and wheels are driven by the last-mentioned output shaft. In the transmission case is further provided a power take-out shaft (hereinafter called PTO shaft) adapted to take out power for driving a working machine connected to the tractor, the input engine power being also transmitted to this PTO shaft at a changed speed, and the power being transmitted to the working machine by the intermediary of the PTO shaft.
In a travelling drive sub-system for driving the wheels is interposed a main clutch so that, upon speed change operations or the like, the travelling drive sub-system can be arbitrarily cut off from the engine. Normally the above-mentioned main clutch is disposed in the proximity of the engine. Here it is desired that the PTO drive sub-system for transmitting power to the working machine via the above-described PTO shaft should operate independently of the aforementioned travelling drive sub-system, that is, regardless of the travelling state of the tractor.
Accordingly, in order to connect the PTO drive sub-system to the engine without the intermediary of the aforementioned main clutch in the travelling drive sub-system, the power transmission system in the tractor was constructed in the prior art, for instance, as shown in FIG. 1 wherein reference numeral E designates an engine mounted on a front portion of a vehicle body, and an output shaft 01 of that engine is disengageably connected to a hollow tubular shaft 04 via a main clutch 03 within a clutch housing 02. The shaft 04 extends into a transmission case 05 in the rear, and within the transmission case 05, a speed-changer consisting of gear trains T.sub.1, . . . . . . , T.sub.4 is formed between the shaft 04 and another shaft 06 disposed adjacently and in parallel thereto. The gears on the side of the shaft 06 in the respective gear trains are loosely fitted around the shaft 06 so as to be freely rotatable and, by selectively connecting any one of these gears to the shaft 06 by means of a selector 07, rotation of the shaft 04 can be transmitted to the shaft 06 at a desired speed change ratio. The shaft 06 is connected via a final shaft 07' to a differential device 08, and wheels are driven by that differential device. In other words, a travelling drive sub-system 09 in this power transmission system is composed of the main clutch 03, the shaft 04, the gear train T, the shaft 06 and the final shaft 07'.
A PTO drive sub-system 010 in this power transmission system is constructed in the following manner. A shaft 011 is directly connected to the output shaft 01 of the engine E, and this shaft 011 extends through the interior of the above-mentioned hollow shaft 04 up to the rear portion of the transmission case 05, and is connected to a shaft 013 via a PTO clutch 012. Between the shaft 013 and another shaft 014 is provided a speed changer similar to the speed changer in the above-described travelling drive sub-system, so that rotation of the shaft 013 may be transmitted at a changed speed to the shaft 014 and further transmitted to a PTO shaft 015. In this way, as the PTO drive sub-system 010 is directly connected to the engine E without the intermediary of the travelling drive sub-system, the PTO shaft can be independently driven by the engine E regardless of the travelling state of the tractor.
In the conventional power transmission system for use in a tractor in the prior art as described above, since the main clutch is provided on the side of the engine, it is necessary to provide a special clutch. In addition it is difficult to extend a directly coupled shaft of an engine that does not pass through the above-mentioned main clutch up to the interior of the transmission case, and to that end, it is necessary to employ a complicated shaft structure such that the shaft 04 in the travelling drive sub-system connected to the main clutch is formed as a hollow shaft and the directly coupled shaft 011 of the engine is made to pass through that hollow shaft as described above.
Furthermore, the above-described directly coupled shaft 011 of the engine must to be formed as a long shaft extending from the engine E up to the rear portion of the transmission case 05.